1. Field of the Invention
The present invention relates to automation systems, and more particularly, to a system and method of automated dispatch and transport of Work-In-Process (WIP) to desired destinations in manufacturing processes.
2. Description of the Related Art
Automated dispatch systems have been used in a variety of manufacturing environments, including semiconductor-manufacturing processes. These systems initiate personnel or automated transport systems to transport materials, such as wafer lots and banks, to desired destinations such as semiconductor fabrication equipment, metrology instruments, or stockers.
In a semiconductor manufacturing environment, many processing equipment installations share stockers. As is known, the stockers stock wafer lots or banks for subsequent processing. Wafer lots are continuously transported to various equipment and stockers, consuming much time. For example, fabrication of an IC chip entails various steps of deposition, cleaning, ion implantation, and etching, each of which must be performed by discrete processing equipment, i.e. a chemical vapor deposition chamber, an ion implantation chamber, an etcher, etc. Efficient an timely transportation of wafer lots or banks is very important in facilitating production.
Manual transportation operation in 12-inch fab facilities has been gradually replaced by automated systems due to the size and space requirements for the 12-inch wafers. Accurate transport of wafer slots and banks is an important aspect of fully automated manufacturing processes. Automated transport systems in manufacturing processes move materials from one site to desired destinations based on instructions from Manufacturing Execution System (MES). Wafer lots are typically input to the transport system using automated equipment which controls flow. Automated equipment is also used to remove wafer lots using the equipment loadport as the exit point, with the transport system and/or removal equipment designed to allow several wafer lots to accumulate near stockers while preventing collisions between adjacent wafer lots. In some applications, wafer lots must be temporarily moved from the transport system to a workstation at one or more locations along the transportation path. Wafer lots are later returned to the transport system, which then transports wafer lots to the next work station or the exit point.
In a conventional automated manufacturing process, a dispatch system is responsible for initiating the transport system or facility to transport materials or work-in-process (WIP) to the desired destination directly. Two dispatch rules (a tool dispatch rule and a lot dispatch rule) are commonly used to dispatch wafer lots. Using known algorithms, the tool dispatch rule determines the target stocker or the processing equipment for given wafer lots or banks, and the lot dispatch rule determines the target wafer lots for given equipment or stocker. These two dispatch rules both attend to many criteria such as priority, yield rate, load balancing, and stability.
Commonly, wafer lots and banks are ideally transported by automated transportation mechanisms to equipment or stockers for manufacturing equipment for an immediately subsequent operation or process step. Wafer lots typically require comprehensive inspection or correction between equipment or process steps due to the delicate nature of the integrated circuits (ICs). When inspection or correction occurs, they are not transported immediately, but instead are held for subsequent processing. Operators hold wafer lots or banks for inspection or correction and record the status thereof for lot hold release, locate forward, locate backward, operation branch or bank hold release in the Manufacturing Execution System (MES), depending on various events. After inspection or correction, wafer lots are released and their current status is recorded as waiting in the MES by operators. Automated transport systems are often associated with interruptions during the entire automated manufacturing process. One problem is that WIP may be abandoned in a stocker after inspection or correction without transport to a desired destination. A second and more serious problem is that transport system may take an unnecessarily long time to transport wafer lots or banks as described because they are isolated from equipment or the nearest stocker. This not only wastes time for transportation, but also decreases equipment utilization.
To address the situation described, operators may trigger transport commands to the dispatching system after inspection or correction. Although the solution is very simple, several problems remain. First, numerous simultaneous triggers may cause traffic conflicts in the transport system if they are not properly scheduled. For instance, when an operator releases a wafer bank containing one thousand lots, one thousand instant transport commands are triggered, potentially seizing the entire automated transport system and crowding other commands. Additionally, the ultimate goal of a fully automated light-out fab is compromised by the system's reliance on operator initiative to trigger transport commands.
In view of the limitations of conventional transport and the solutions described above, a need exists for a system and method of efficient material transport that effectively reduces the transport time and improves equipment utilization, and which also avoids traffic conflict.